Acids and bases Flashcards
What is the Brønsted-Lowry definition of an acid
A proton donor
What is the Brønsted-Lowry definition of a base
A proton acceptor
What is the hydroxonium ion and why does it exist
- The hydroxonium ion is H30 +
- A proton contains no electrons and chemists realised that this cannot exist independently in aqueous solution.
- This is because its diameter is extremely small (10^-15 m) which means it has an intense electric field.
- Instead H+ associates with water molecules and forms the hydroxonium ion H30+
- However, for simplicity when writing equations, the less precise H+ is used.
Why are the Brønsted-Lowry definitions of an acid and base better than ones which are based on PH
A compound can act as both an acid and a base depending on the reaction it is in.
What can the H+ ion bond with
Species that have a lone pair of electrons as H+ does not have any electrons.
What state does water exist in (in terms of acid-base reactions)
It slightly ionises so that an equilibrium is set up between H20 and H+ and OH-
What are the two equations that represent the slight ionisation of water and which is more accurate
1) H20 (l) <=> H+ (aq) + OH- (aq)
2) H20 (l) <=> H3O+ (aq) + OH- (aq)
- The second equation is more accurate because it takes into account that H+ cannot exist on its own
- And, it emphasises that the ionisation of water is an acid-base reaction where one molecule of water donates a proton to another.
What is Kw
- A modified equilibrium constant called the ionic product of water.
- The concentration of water is constant so Kc for the partial ionisation of water is multiplied by the concentration of water to give Kw.
- Kw= [H+(aq)][OH-(aq)]
What is Kw equal to at 298K
1 x 10^-14 mol^2dm^-6
What is the expression for Kw
Kw = [H+(aq)][OH-(aq)]
What does the acidity of a solution depend on
The concentration of H+ ions
What is the equation for the PH of a solution
PH= -log10[H+(aq)]
What two key things do you need to remember about the PH scale
- The smaller the PH, the greater the concentration of H+ (aq)
- A difference of one PH number means a tenfold difference in [H+] so that, for example, pH2 has ten times the H+ concentration as pH3.
Why is the pH equation pH=-log10[H+(aq)]
- Using the logarithm of the concentration gets rid of awkward small numbers (such as 10^-13) which occur because the concentration of H+ ions in most aqueous solutions is very small.
- The minus sign makes all pH values positive (because the logs of numbers less than one are negative).
How do you prove that pH seven is neutral pH (pH if water) at room temperature
- Kw for water at room temp is 1 x 10^-14
- This can be used to deduce that the concentration of H+ ions is half of this: 1 x 10^-7 (using the expression for Kw)
- Substitute this into the pH equation pH= -log10[H+]
- This gives you 7.
How does pH measure alkalinity despite only taking into account the concentration of H+ ions
Because as the concentration of H+ ions decreases, the concentration of OH- ions increases and vice versa.
What is a strong acid
Acids that dissociate completely
What is a weak acid
An acid that only partially dissociates/ionises
What is Ka and how is it formed
- Ka is the acid dissociation constant
- It is formed in the same way as Kc is.
-It is Kc for the equilibrium of an acid that only partially dissociates
How do you calculate the pH of an acid that only partially dissociates
- Write an expression for Ka
- Find the value of Ka.
- Use this to work out the concentration of H+ ions.
- Substitute this into the pH equation to find pH
Why is a pH metre useful
It is more accurate than an indicator
How do you calibrate a pH metre
Place the probe in a buffer solution of known pH
What is the equivalence point in a titration
-The point at which a sufficient base has been added to just neutralise the acid or vice versa
- This occurs at the volume when exactly the same number of moles of hydrogen ions (or hydroxide ions) have been added as there are moles of hydroxide ions (or hydrogen ions).
How do you identify the equivalence point on a graph showing pH changes during a titration
- It is the point at which there is a large and rapid change in pH.
- The curve will often look almost vertical.
- It is the equivalence point even if it is not centered around pH7
Why is the equivalence point important
It can be used to work out the concentration of the unknown acid or base.
What is the end point of a titration
The volume of acid or alkali added when the indicator just changes colour.
Why is it important that the right indicator is chosen for a titration
If you don’t choose the right indicator, the equivalence point and the endpoint may not give the same answer.
What properties does a suitable indicator for a titration need
- The colour change must be sharp rather than gradual at the end point: no more than one drop of acid (or alkali) is needed to give a complete colour change.
- The end point given by the indicator must be the same as the equivalence point otherwise the titration will give the wrong answer.
- The indicator should give a distinct colour change.
What is the advantage of using phenolphthalein as an indicator as opposed to methyl orange
The colourless to pink change of phenolphthalein is easier to see than the red to yellow of methyl orange- the colour change is more distinct.
Why is universal indicator not suitable for any titration
Its colour changes are gradual
How do you determine the best indicator from a pH graph of a titration
A suitable indicator will change at the equivalence point- its should overlap the vertical section of the graph at some point (it does not need to cover the whole vertical section just a part of it).
What is the half-neutralisation point
The point halfway between zero and the equivalence point.
Why is knowing the half-neutralisation point useful
- The knowledge that you can add acid (or base) up to this point with the certainty that the pH will change very little is relevant to the theory of buffers.
- It also allows you to find the pKa of a weak acid.
What is a buffer solution
Buffers are solutions that can resist changes in pH (when a small amount of acid or alkali is added to them).
Summarise how buffers work
- Buffers are designed to keep the concentration of hydrogen ions and hydroxide ions in a solution almost unchanged.
- They are based on an equilibrium reaction which will move in the direction to remove the additional ions.
What is an acidic buffer
An acidic buffer is made from a mixture of a weak acid and a soluble salt of that acid. It will maintain a pH of below 7.
What is the pH at half neutralisation point
At half neutralisation point, pH=pKa
What is a basic buffer
A basic buffer is made form a weak base and a salt of that base. It maintains a pH at above 7.
What is the purpose of the weak acid part of an acidic buffer
To act as a source of HA (undissociated acid) which removes any added OH- ions.
What is the purpose of the salt part of an acidic buffer
To act as a source of A- ions which remove any added H+ ions.
What is a way of using neutralisation to make an acidic buffer
- Neutralise some of the weak acid with an alkali to the half neutralisation point at which the pH is equal to pKa.
- This is a useful buffer because it is equally efficient as resisting a change in pH whether it is an acid or alkali added.
What is the purpose of the weak base component of a basic buffer
To remove any H+ ions added
What is the purspose of the salt component of a basic buffer
To remove any OH- ions that are added
What is a key example of a system involving a buffer and what is the pH maintained at
- Blood contains a buffer
- The pH is maintained at approximately 7.4
- A change of as little as 0.5 of a pH may be fatal.
What is the most important mechanism by which blood is buffered to a pH of 7.4 by
H+ (aq) + HCO3- (aq) <=> CO2 (aq) + H2O (l).
What is pKa
-log10Ka
Over what pH range does methyl orange change colour, what is its colour change and what type of titration is it useful for
- methyl orange changes colour between pH 3-4.5
- it is red at low pHs and orange/yellow at high pHs
- it is useful for strong acid/strong base and strong acid/weak base titrations
Over what pH range does phenolphalein change, what is the colour change and what type of titration is it useful for
- phenolphalein changes between pH 8.2-10
- it changes from colourless at lower pHs to pink at higher pHs
- it is useful for weak acid/strong base titrations
What assumptions do you make when working with buffer solutions
- No HA dissociates
- All of the salt XA- dissociates
- The salt does not change the volume of the buffer solution (if dissolved as a solid)
- solution salts do change the volume
